Nowadays, repeater systems are constituted to serve a wide frequency band, for example a complete 3GPP band. Herein, a frequency band typically contains multiple carriers carrying information according to different communication technologies and standards such as GSM, UMTS, LTE or the like.
The power efficiency of a (high-power) repeater system depends on the efficiency of a power amplifier, making use for example of a transistor. A power amplifier in this context generally is used in the linear regime in which the output power linearly relates to the input power. The closer the power amplifier is operated to its saturation point, the more non-linear distortions will occur in the amplified signal, leading to unwanted intermodulation products. A power amplifier hence generally is operated with a significant backoff from its saturation point, meaning that the power amplifier's maximum output power level is reduced in a way such that the entire signal lies within the linear region of the power amplifier's transfer curve. Because the backoff reduces the efficiency of the power amplifier (namely the power amplifier's ability to convert the DC supply power into RF energy), it however is desired to keep the backoff as small as possible.
The backoff generally should be chosen according to the peak-to-average ratio (PAR) of the signal to be amplified. If signal peaks exceed the saturation point of the amplifier, this may lead to a clipping of the peaks, causing intermodulation products and adding to the noise, resulting in an increased error rate for a radio transmission.
Generally, the quality of a broadband RF signals quantified by the so-called error vector magnitude value (in short: EVM). The error vector magnitude represents a measure for the deviation of a symbol from its ideal constellation point. The error vector magnitude serves as a characteristic value for the signal amplification and the noise of a receiver and may be indicated in dB or percent.
For different radio access technologies, different permissible limits for the error vector magnitude exist. For example, in UMTS (using a 16-QAM constellation) an error vector magnitude limit of 12.5% must not be exceeded. UMTS signals hence must have an EVM smaller than 12.5%. In another example, in LTE (using a 64-QAM constellation) an error vector magnitude limit of 8% exists.
An input signal generated for example by a base station and received at a receiver in the downlink direction (towards a user equipment) in general will comprise a non-zero error vector magnitude. When passing the repeater system, the repeater system will additionally contribute to the error vector magnitude of the signal. If a signal comprises for example an error vector magnitude of 8% at the input of the repeater system and if the repeater system adds another 8% to the error vector magnitude, the resulting error vector magnitude will be about 11.3%, assuming that the error vector magnitude contributions are uncorrelated. This would result in an unacceptable EVM for a 64-QAM transmission (for example in LTE), while the permissible limit for 16-QAM transmissions (in UMTS) would not be reached.